What's the best way to reliably end a call while saying a dynamic farewell message (either negative or positive)? If I turn on the endCall function, even if I mention multiple times within the prompt to first say my custom farewell message and only then call the endCall function, the assistant still immediately calls endCall and says the default "Goodbye" provided by the Vapi endCallMessage prop.

Set a custom endCallMessage, clearly instruct the assistant to speak the “Goodbye” before calling endCall, and if needed, manage it in two steps: speak first, then trigger endCall.